The present invention relates to optical communication systems and more particularly to amplification in optical communication systems.
The explosion of communication services, ranging from video teleconferencing to electronic commerce, has spawned a new era of personal and business interactions. As evident in the rapid growth of Internet traffic, consumers and businesses have embraced broadband services, viewing them as a necessity. However, this enormous growth in traffic challenges the telecommunication industry to develop technology that will greatly expand the bandwidth of existing communication systems. Further improvements in optical communications hold great promise to meet continuing demands for greater and greater bandwidth.
Wavelength Division Multiplexing (WDM) technology, in particular Dense WDM (DWDM) techniques, permits the concurrent transmission of multiple channels over a common optical fiber. The advent of Erbium Doped Fiber Amplifiers (EDFAs) has accelerated the development of WDM systems by providing a cost-effective optical amplifier that is transparent to data rate and format. An EDFA amplifies all the wavelengths simultaneously, enabling the composite optical signals to travel large distances (e.g., 600 km or greater) without regeneration.
One of the principal limitations of EDFA technology is limited bandwidth. Discrete and distributed Raman amplifiers have been developed to overcome this limitation. They provide very high gain across a wide range of wavelengths. Moreover, discrete and distributed Raman amplifiers increase the distance between optical regeneration points, while allowing closer channel spacing. The operation of Raman amplifiers involves transmitting high-power laser pump energy down a fiber. The pump energy amplifies the WDM signal.
The performance of Raman amplifiers in DWDM systems is limited by various impairments. One such impairment is four-wave mixing, a common detriment to optical communication system performance. If three wavelength components of a DWDM signal located at the optical frequencies f1, f2, and f3 are being amplified, non-linear effects will cause generation of an undesired fourth component at ffwm=f1+f2−f3. This undesired fourth component is a four-wave mixing product. The four-wave mixing product represents a noise-like impairment that can affect reception of a WDM channel at or near ffwm.
Suppressing the generation of four-wave mixing products has been a key concern in the design of Raman amplifiers, both discrete and distributed. In particular, the desire to limit four-wave mixing effects has led Raman amplifier designers to inject pump energy into a fiber exclusively in a counter-propagating direction relative to the propagation direction of the signal to be amplified. Unfortunately, such an approach also concentrates the amplification effects towards the end of the fiber, limiting the signal to noise ratio performance of the Raman amplifier.
What is needed are systems and methods for improving both four-wave mixing product suppression and signal to noise ratio in Raman amplifiers.